FM transmitter antenna questions...

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51fordf2

New Member
Hi, all...newb here.

I have an FM transmitter, that I am planning on using for a computerized Christmas light display next winter. I'm pretty sure it's 100 - 200 mw at best. I can reach the street, but that's about it. I need it to go maybe 150 feet more, so that more than one car can hear it at a time. It has a telescopic antenna, and it's only attached at one point on the circuit board. I am broadcasting on 88.1 Mhz, so I believe the antenna should be about 52 inches, but it appears to be about 1/2 that, or so. Does that sound right?

Anyway, my question is, I'd like to replace the antenna with a dipole, but with just one attachment to the board, is there any way to do that? I don't know enough about transmitters to know why there's only one connection to the antenna - basically a single wire - whereas "most" I have seen, have two connections, and an RG58 coax can be used, with a dipole or similar.

Is there anything I can do to increase range just a bit? For me, simplest would be something to do with the antenna...

Roger

colin55

Well-Known Member
I have designed FM transmitter circuits that are less than 50mW and one customer reached more than 20 miles in a line-of-sight from a mountain-top.
Most of our 30mW devices reach 400 yards, so anything like 100mW will certainly reach 1,000 yards with a 170cm antenna that is placed vertical and as high as possible.

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duffy

Well-Known Member
The other connection would be the ground on the board. You can use that for a ground plane under the antenna, that might give you the signal boost you need.

51fordf2

New Member
Colin and Duffy - thanks, I'm now part-way there, but brings to mind a few more questions. I am an electronics technician, but I have never played with radio at all, forgive me if they seem pretty basic...

Colin - you mention an antenna 170 cm but placed high. All I am familiar with is the dipole. As currently there is only one connection to the board, how would the antenna and transmission line be configured? The telescopic antenna currently is connected directly to the board, by a screw, and sticks out the case. I would need to change that to a wire of some sort, I assume, which is the easy part. What would the antenna itself look like? Would it be like a "T", with the single wire connection in the middle? It would still look something like a dipole? Do you know of any examples on the web, that would point me to a picture or very good description?

Also, the 170 cm...is that determined by the frequency I am planning on broadcasting on? I know the wave is 468/F, which would give me 161.9 cm for 88.1 Mhz, so is 170 about the middle of the 88-108 FM band? If I am "dedicated" to the 88.1, would I want to be closer to the 162 cm?

Duffy - If I took a piece of RG58, and connected the middle conductor to the current connection, and the shield to someplace on ground plane, then out to a dipole, with a balun in-line, would that accomplish what you are saying? And if so, could there be any dangers to the circuit if I did that? If so, what might they be? I would much prefer to use the coax, if at all possible, with a dipole, which, in my experience, are very good, compared to just the basic "wire" antenna. I just don't want to do something that can harm the transmitter.

Thanks again, gents!

Roger

flat5

Member
Can you post a clear copy of the circuit?
Someone here may be able to design the correct balum at the pc board. Then you could use 50 or 75 ohm coax to a 75 ohm dipole.

You might try using 300 twin lead from the ant. and ground at the pcb and run it to the center of a 300 ohm dipole as high and clear as possible and aimed at the street. It would be pretty simple and cheap to try.

and yes I know 300 ohm twin lead is for a balanced line

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duffy

Well-Known Member
Actually, I would avoid the balun, coax, and RF connectors completely. You always lose some signal in those, and you need all the signal you got.

I would suggest mounting the transmitter in a waterproof enclosure outside, run the audio and power to it. Connect the antenna and ground wires directly. Stick it on a pole, use a quarter-wave vertical antenna, string some wires down for a ground plane out from the bottom of the transmitter in a radiating pattern like a tree. Maybe put lights on the ground plane wires, make it another decoration.

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51fordf2

New Member
Can you post a clear copy of the circuit?
Someone here may be able to design the correct balum at the pc board. Then you could use 50 or 75 ohm coax to a 75 ohm dipole.

You might try using 300 twin lead from the ant. and ground at the pcb and run it to the center of a 300 ohm dipole as high and clear as possible and aimed at the street. It would be pretty simple and cheap to try.

and yes I know 300 ohm twin lead is for a balanced line
I am including some pictures, if that's what you are looking for.

As for the 300 twin lead, I would put one leg to where the antenna currently is, and the other attached to the groundplane on the board? Would I make it a closed loop, or open-ended on the dipole? I'm not really sure what you mean by a "300 ohm dipole".

Anyway, here's the pics:

Thanks again, hope I'm not being a pain!

R

duffy

Well-Known Member
WTF? Is that a METAL case? Wonder what kind of SWR you are getting with that sad excuse for a feed-through...

51fordf2

New Member
Yup, metal...this was originally an FM transmitter for use inside one of the Jazzercise places, to transmit to the gals, wearing Walkman type headsets. Can't find any information on it. However, the signal that it DOES put out, is very clean and clear, that's why I want to use it, if I can.

I just measured the antenna that's on it - 19 inches. I plan on using 88.1/88.3, which 1/4 wave is about 32 inches, so can I increase things with changing the antenna, that I can adjust to 32", or even a half-wave at 64"? Again, I am completely in the dark when it comes to this stuff! My background is in robotics, controls, etc.

Oh yeah, just ordered an SWR meter, but, don't I need coax to use it? My dipole I have has BNC connectors, RG58 coax.

Thanks,

R

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audioguru

Well-Known Member
The lousy old BA1404 FM stereo transmitter IC had such poor performance that it was discontinued and replaced by a range of modern ICs.

duffy

Well-Known Member
I'm concerned about that feed-through arrangement, where that whip antenna goes through the hole in the metal case? That doesn't look even close to being right to me, I bet you are losing signal there. Since the company wasn't too concerned about giving you the right size whip antenna, they probably weren't too concerned about that feedthrough arrangement, either, and just kinda "eyeballed" it.

My buddy John at work suggested mounting an SO239 connector where that hole is, and connecting the antenna to that. Also he says to make sure you tune those radials to the same 32" length.

Even with the unimpressive silicon, I think you could enhance the signal strength a great deal with only modest improvements to the antenna and the way the signal is getting out of the box.

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51fordf2

New Member
I'm concerned about that feed-through arrangement, where that whip antenna goes through the hole in the metal case? That doesn't look even close to being right to me, I bet you are losing signal there. Since the company wasn't too concerned about giving you the right size whip antenna, they probably weren't too concerned about that feedthrough arrangement, either, and just kinda "eyeballed" it.

My buddy John at work suggested mounting an SO239 connector where that hole is, and connecting the antenna to that. Also he says to make sure you tune those radials to the same 32" length.

Even with the unimpressive silicon, I think you could enhance the signal strength a great deal with only modest improvements to the antenna and the way the signal is getting out of the box.
This started out, because I have a BNC connector that I could use where the hole is, but when I realized I only had one antenna connection to the board, I started looking into how to make it work. I figured if I could use the current antenna connection for the middle conductor on the coax, and the shield to the ground on the board, then I could set up my dipole, and at least get some idea where I'm at with this.

It boils down to me having made the (stupid, STUPID!) mistake of buying not one, but TWO transmitter's from China - one supposedly .5 watt, and the other 5 watt. The 5 watt was for "playing" with (please, quell the urge to tell me about the law, etc...). Both have so much noise, and drift that I am sending them back. Of course, NOW they have trouble with english...

So, I am really pretty much stuck with making this one do what I need it to do. I don't need much more out of it, maybe 50% more distance at the most, which I think is pretty do-able. And I think a decent antenna would go a LONG way towards getting me there. I am spending most of my time with building the control boards, and solid state relays for the lighting display, and really don't want to have to learn a whole new field, just to get my music where it needs to be. Just not that interested in the RF side of the hobby...

Thanks,

Roger

birdman0_o

Your being difficult about not buying a modern one, they're really cheap and people will actually enjoy the music without needing to wonder why the signal is so weak. If you cant bring something to you, go to it! Why not just bring the transmitter closer to the road so that it's within range? Or, once again just get a new one, if your spending all this money on lights and controllers, why not spend the extra $for something that works? 51fordf2 New Member Your being difficult about not buying a modern one, they're really cheap and people will actually enjoy the music without needing to wonder why the signal is so weak. If you cant bring something to you, go to it! Why not just bring the transmitter closer to the road so that it's within range? Or, once again just get a new one, if your spending all this money on lights and controllers, why not spend the extra$ for something that works?
So, at the risk of sounding flippant, are you saying that this WON'T work, or why bother, or that I just like being difficult?

I currently have serveral hundred dollars in "junk", that I'm not confident that I will get paid back, upon return of them. As for the controllers, they are all home-built, at very little expense. I am running 448 channels, at 1/10th the cost of a commercial 32 channel controller. So the eye has been on economically doing this. It is solely for donations to the local charities. It's just something I do. I hate the idea of spending another \$150 for a transmitter that I'll only use several months a year, and is 1/3 of the cost that I have in the whole operation, excluding the lights (many are donated).

As I said, this works excellent, with the exception of the range. It is crystal clear, at the street, but the difference from clear, to non-existent is about 10 feet. I really don't want to move it outside, unless I absolutely have to, and that is why I am trying to explore all other avenues before I resort to that. It has already been pointed out that it is an inefficient way for the antenna to exit the box as well as the antenna is not truly a decent length. Perhaps mods will give me the distance I desire which is probably about 50 - 100 feet more than I have at present. My time is cheap, so experimentation is not costing me anything. I don't have any problem moving the antenna outdoors, I just don't want the transmitter out there. So if I can figure out a way, to do that, I'd be happy. But, I don't know the effects of just lengthening a lead to an antenna, etc, however, I am familiar with the dipole, and coax, and that's why I was leaning towards that, if it was plausible.

I am still hoping for an answer about damage if I connect the shield of the coax to the board, as well as some direction as to building a single-wire antenna, that I can extend outside. I really don't understand that, what it would look like, be made, etc.

Unless, of course, someone has a transmitter that would do me, that is willing to donate?? Not really. Guess just a "little" flippant...

R

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51fordf2

New Member
Addendum: I have the transmitter out of the metal box (with some difficulty). In series, just before the antenna, there's a .001μF cap in series with a 100 Ω. I am assuming this is an attenuator circuit? Perhaps some fiddling with these?

Perhaps I am aggravating - I don't intend to be. I just think this can be done, as I have seen people on my other forum, use a cheap Belkin Tunecast II, with a LONG piece of wire, go for blocks! I don't think I am asking too much from the transmitter that I have, and indeed, if the sound wasn't so good out of it, I would be tempted to search elsewhere. But, it IS very good, just a tad short ranged.

R

colin55

Well-Known Member
This is the best advice so far.
Actually, I would avoid the balun, coax, and RF connectors completely. You always lose some signal in those, and you need all the signal you got.
It seems the 100mW output is not being achieved. I have a simple "power meter" circuit or "field strength meter" circuit that gives you some idea of the output, when you reference the output to other transmitters.
Put the transmitter on the top shelf of a cupboard and connect a piece of hook-up flex to the antenna terminal and stretch it upwards. The longer the antenna the better and the length can be cut to half-wave. The simple addition of a longer antenna is going to have a greater effect than all the other changes.
Does the circuit have an RF amplifier stage after the output of the BA1404 chip?
Does the board connect to the case?
You should be getting at least 200metres (walkmen are not very sensitive).
Connect the antenna directly to the collector of the output transistor.

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51fordf2

New Member
This is the best advice so far.
Put the transmitter on the top shelf of a cupboard and connect a piece of hook-up flex to the antenna terminal and stretch it upwards. The longer the antenna the better and the length can be cut to half-wave. The simple addition of a longer antenna is going to have a greater effect than all the other changes.
I'll try that first, although I'm a little confused - you said "the longer the better, and the length can be cut to half-wave". Did you mean that I can start longer, to see if it gets better range, then tune it by cutting to half-wave?

Does the circuit have an RF amplifier stage after the output of the BA1404 chip?
Not sure. A bit above me. There is a transistor off of it...

Does the board connect to the case?
Yes, but only to the copper pours, I guess that would be the "ground plane"?

You should be getting at least 200metres (walkmen are not very sensitive).
Connect the antenna directly to the collector of the output transistor.
That's what I thought - that I should be getting a little further out with it, than I am. And as I said, I'm not getting any static or anything, nice clear signal, just shorter range than I would expect.

Thanks, I'll try everything you have mentioned. Connecting the antenna directly to C of the output transistor, is effectively removing both the cap and resistor from the circuit - then that is an attenuator circuit?

I appreciate all the help, folks. Ain't the internet great! Hopefully, I'll be able to return the favors someday, in areas that I am well versed in...

R

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colin55

Well-Known Member
You really don't have to "trim" the antenna, but a full-wave will be better than half-wave and any of the shorter lengths. But a full-wave is about 3.3 metres, so all the kits I produce use a 170cm antenna, and the problem is solved.
It looks like the kit has attenuated to signal to near zero. The world record (30 years ago) was 300 miles from 300 microwatts. That’s one million miles for 1 watt.

duffy

Well-Known Member
Yes, that's an attenuator, cut that resistor down and it will probably increase the signal.

The case should be grounded (make sure it is, probably on one of those posts, be sure they didn't paint over it) and that would be the shield on the coax, and no - it won't hurt anything.

I think you might also get some improvement in gain if you just drill that antenna hole out a little larger - darn thing just LOOKS too small to me.

51fordf2

New Member
You really don't have to "trim" the antenna, but a full-wave will be better than half-wave and any of the shorter lengths. But a full-wave is about 3.3 metres, so all the kits I produce use a 170cm antenna, and the problem is solved.
It looks like the kit has attenuated to signal to near zero. The world record (30 years ago) was 300 miles from 300 microwatts. That’s one million miles for 1 watt.
I have plenty of room - when I'm "done" the transmitter will be on the road side of the house, against the wall. I could easily put a full-wave there. I have always used 12 or 14 gauge Romex (house wiring copper) uninsulated. From what I understand, the insulation doesn't matter, so can leave it on. Is there a better wire to use? Bigger cross section? Smaller cross section? The larger has more surface area, so maybe that's better?

Yes, that's an attenuator, cut that resistor down and it will probably increase the signal.
At this point, with a half-wave piece of the Romex, uninsulated, I gained probably 100 feet of range - I can now get to the corner, as well as 1/2 way down the next block. That is sufficient, but I also picked up some "noise" towards the end of the range, when before it was pretty abrupt, between signal and loss of signal. Not important. I did find that I LOST range, when I bypassed the cap and resistor - I didn't try just bypassing the resistor. It will be easy enough to test, by putting a resistor or two in parallel.

The case should be grounded (make sure it is, probably on one of those posts, be sure they didn't paint over it) and that would be the shield on the coax, and no - it won't hurt anything.
The case is grounded, to the board, but it doesn't look like it's connected to circuit ground, just to the copper pours. I can check. Should it be to circuit ground, or just the ground plane, if it isn't circuit ground? And, when you say the shield, would that be circuit ground, or just to the board?

I think you might also get some improvement in gain if you just drill that antenna hole out a little larger - darn thing just LOOKS too small to me.
That was one of my concerns, as well, as there is very little clearance between the case, and the antenna stand-off. Plus, there's a little play in the antenna, and I was concerned about it touching inadvertently. Although it was mentioned that there would be some loss, I still like the idea of making a removable antenna, with a BNC connector.

Doesn't a dipole add gain? Since I know that I can increase the range, just with the wire, couldn't I experiment, and see what it will do with the dipole? And if nothing else, I can solder a BNC to the wire, and still use that. I can live with 1/2 way down the block, but if I could squeeze just a little more, I'd be a ecstatic! I wouldn't mind the viewers having a little bit of music, as they drive away, than abruptly cutting out when they leave from right in front.

Here's a link to one such display, for those interested (make sure you have sound on, and check out his other videos) - this one is particularly nice, and very artfully done. Mine has some similar items, but has a lot more lights, and a lot more "independent" action.

Holdman Christmas Lights

Thanks,

Roger

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